1. Field of the Invention
This invention relates to a method of processing nickel-titanium-base shape-memory alloys to substantially suppress the two-way effect and to a composite structure including a nickel-titanium-base shape-memory alloy with the two-way effect substantially suppressed.
2. Discussion of the Prior Art
Materials, both organic and metallic, capable of possessing shape memory are well known. An article made of such materials can be deformed from an original, heat-stable configuration to a second, heat-unstable configuration. The article is said to have shape memory for the reason that, upon the application of heat alone, it can be caused to revert or attempt to revert from its heat-unstable configuration to its original, heat-stable configuration, i.e., it "remembers" its original shape.
Among metallic alloys the ability to possess shape memory is a result of the fact that the alloy undergoes a reversible transformation from an austenitic state to a martensitic state with a change of temperature. Also, the alloy is considerably stronger in its austenitic state than in its martensitic state. This transformation is sometimes referred to as a thermoelastic martensitic transformation. An article made from such an alloy, for example, a hollow sleeve, is easily deformed from its original configuration to a new configuration when cooled below the temperature at which the alloy is transformed from the austenitic state to the martensitic state. The temperature at which this transformation begins is usually referred to as M.sub.s and the temperature at which it finishes M.sub.f. When an article thus deformed is warmed to the temperature at which the alloy starts to revert back to austenite, referred to as A.sub.s (A.sub.f being the temperature at which the reversion is complete), the deformed object will begin to return to its original configuration.
Alloys of nickel and titanium have been demonstrated to have shape-memory properties which render them highly useful in a variety of applications.
Shape-memory alloys (SMAs) have found use in recent years in, for example, pipe couplings (such as are described in U.S. Pat. Nos. 4,035,007 and 4,198,081 to Harrison and Jervis), electrical connectors (such as are described in U.S. Pat. No. 3,740,839 to Otte & Fischer), switches (such as are described in U.S. Pat. No. 4,205,293), actuators, etc., the disclosures of which are incorporated hereby by reference.
Various proposals have also been made to employ shape-memory alloys in the medical field. For example, U.S. Pat. No. 3,620,212 to Fannon et al. proposes the use of an SMA intrauterine contraceptive device, U.S. Pat. No. 3,786,806 to Johnson et al. proposes the use of an SMA bone plate, U.S. Pat. No. 3,890,977 to Wilson proposes the use of an SMA element to bend a catheter or cannula, etc., the disclosures of which are incorporated herein by reference.
These medical SMA devices rely on the property of shape memory to achieve their desired effects. That is to say, they rely on the fact that when an SMA element is cooled to its martensitic state and is subsequently deformed, it will retain its new shape; but when it is warmed to its austenitic state, the original shape will be recovered.
The shape change occurring suddenly and only through the influence of temperature is described as the one-way effect because the shape prior to raising the temperature is not regained upon subsequently decreasing the temperature but must first be reformed mechanically. In some cases, however, upon subsequent thermal cycling a purely thermally-dependent shape reversibility is observed which is described as the two-way effect. In applications such as thermoelectric switches, for example as described in U.S. Pat. No. 4,205,293, the two-way effect is useful. In other applications, however, it is desired to suppress the two-way effect, for example, in couplings. Thus, on heating and making a coupling with an alloy whose transformation temperature is above room temperature, the two-way effect causes the coupling to become loose on cooling back to room temperature.
Clearly, therefore, it is desirable to develop processing which will substantially suppress the two-way effect in nickel-titanium-base shape-memory alloys.
Methods of achieving cyclic stability are known in the art, as from U.S. Pat. Nos. 3,948,688, 3,652,969 and 3,953,253. However, these patents suffer from the disadvantage that thermal cycling under load of the component is required and they do not suppress the two-way effect. Also, it is desirable to achieve cyclic stability in a method that can be applied to the semi-finished product, for example, bar, wire or sheet, during the normal.manufacturing procedure and thereby provide significant cost savings.
U.S. Pat. No. 4,283,233 describes a process for varying the shape change temperature range (TTR) of Nitinol (nickel-titanium based) alloys by selecting the final annealing conditions. Prior to the annealing step the alloy is cold worked to bring it to a convenient size and shape and to remove any prior shape-memory effect which may be present in the alloy. The material is then formed into its permanent shape, restrained in this permanent shape and annealed under restraint. This procedure does not substantially suppress the two-way effect.
It is known that cold work can impart interesting effects to nickel-titanium-base alloys (for example, see T. Tadaki and C. M. Wayman, Scripta Metall., Vol. 14, P. 911, 1980), and the stress-strain curves at room temperature after cold work the annealing at temperatures between 300.degree. C. and 950.degree. C. have been reported; see O. Mercier and E. Torok, International Conference on Martensitic Transformations (ICOMAT), Leuven, 1982, P. C4-267. Also, work by Otsuka, for example, S. Miyazaki, Y. Ohmi, K. Otsuka and Y. Susuki, ICOMAT, Leuven, 1982, P. C4-255 and K. Otsuka and K. Shimizu, International Summer Course on Martensitic Transformations, Leuven, 1982, has shown that pseudoelstic effects are improved by cold working followed by annealing at 300.degree. C.
It is therefore highly desirable to develop a method of processing nickel-titanium-base shape-memory alloys to substantially suppress the two-way effect and a composite structure including a nickel-titanium-base shape-memory alloy with the two-way effect substantially suppressed.